<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
        var i = 1
        console.log(++i); // 2   +在前面 先加再输出
        
        var j = 1
        console.log(j++); // 1   +在后面先先返回再加
        console.log(j); //返回后此时为2
    </script>
</body>
</html>